This invention relates to the manufacture of shadow mask type color television picture tubes. More particularly, it relates to a method for forming a substantially continuous line screen in a color television picture tube having an interrupted-aperture shadow mask.
Shadow mask picture tubes usually include a screen of red, green and blue emitting phosphor lines or dots, electron gun means for exciting the screen and a shadow mask interposed between the gun means and the screen. The shadow mask is a thin multiapertured sheet of metal precisely disposed adjacent the screen so that the mask apertures are systematically related to the phosphor lines or dots.
In one prior art process for forming each color array of phosphor lines or dots on a viewing faceplate within the tube, the inner surface of the faceplate is coated with a mixture of phosphor particles adapted to emit light of one of the three colors, e.g., blue, and a photosensitive binder. A light field is projected from a point source through the shadow mask apertures and onto the coating so that the shadow mask functions as a photographic master. The exposed coating is subsequently developed to produce phosphor elements of the first phosphor; e.g., blue emitting lines or dots. The process is repeated for the green-emitting phosphor and for the red-emitting phosphor utilizing the same shadow mask but repositioning the point source of light for each exposure. A more complete description of a prior art process for forming a picture tube screen can be found in U.S. Pat. No. 2,625,734 issued to me on Jan. 20, 1953.
When the foregoing screen printing process is utilized with a shadow mask having a plurality of rows of apertures, wherein the apertures in each row are separated by webs, to form a line screen, each line of a particular phosphor color becomes a series of spaced dashes on the tube faceplate because of the shadowing effect of the webs. The length, h, of each phosphor dash is determined by the following equation wherein: L is the distance from the point source to the screen; q is the distance from the shadow mask to the screen; and B is the length of an associated shadow mask slit. EQU h = B L/(L-q)
If the electron beam emitted by the electron gun followed the same path as the light used to form the phosphor lines, the length of a phosphor dash, as given by the preceeding equation, would be adequate for attaining optimum picture brightness from this type of tube. However, the electron beam in a picture tube does not follow the path of the light used in the screen printing process. Instead, as known in the art, during beam deflection away from the central axis of the tube, the effective beam-deflection center moves toward the screen as deflection is increased. This effective shift of the deflection plane results in the electron beam passing through the shadow mask and striking the screen at a slightly different angle than did the light that was used to produce the screen. Therefore, portions of the electron beam will not land on the phosphor dashes produced by the preceding process but instead will land on the shadowed areas lying in between the dashes of one row. Since it is desirable to maximize tube brightness, the nonuse or loss of the electrons that do not strike the phosphor dashes is unacceptable.
Therefore, to attain increased tube brightness, while maintaining the advantage of accurate registry between apertures and lines that occurs when the shadow mask is used as a photographic master for line formation, it is necessary to either extend the length of each phosphor dash into the area between dashes of each row or, preferably, to form continuous phosphor lines.